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Diazo ester insertion in N-H bonds of amino acid derivatives and insulin catalyzed by water-soluble iron
and ruthenium porphyrin complexes (FeTSPPCl) as application of carbenoid transfer in aqueous media
Hassan F. Srour, Paul Le Maux, Soizic Chevance, Daniel Carrié, Nicolas Le Yondre, Gérard Simonneaux
To cite this version:
Hassan F. Srour, Paul Le Maux, Soizic Chevance, Daniel Carrié, Nicolas Le Yondre, et al.. Diazo ester insertion in N-H bonds of amino acid derivatives and insulin catalyzed by water-soluble iron and ruthe- nium porphyrin complexes (FeTSPPCl) as application of carbenoid transfer in aqueous media. Journal of Molecular Catalysis A: Chemical, Elsevier, 2015, 407, pp.194-203. �10.1016/j.molcata.2015.06.028�.
�hal-01174902�
Diazo ester insertion in N-H bonds of amino acid derivatives and insulin catalyzed by water-soluble iron and ruthenium porphyrin complexes
(FeTSPPCl) as application of carbenoid transfer in aqueous media
Hassan F. Sroura, Paul Le Mauxa, Soizic Chevancea, Daniel Carriéa , Nicolas Le Yondreb and Gérard Simonneauxa,*
aUMR 6226 Institut Sciences Chimiques de Rennes, Campus de Beaulieu, Université de Rennes 1, 35042 Rennes, France
bCRMPO, Campus de Beaulieu, Université de Rennes 1, 35042 Rennes, France
Corresponding author G. Simonneaux. Tel. 33(0)223236285; Fax: 33(0)223235637 Email: gerard.simonneaux@univ-rennes1.fr
Graphical abstract
Diazo ester insertion in N-H bonds of amino acid derivatives and insulin catalyzed by water-soluble iron and ruthenium porphyrin complexes as
application of carbenoid transfer in aqueous media
Highlights
• we report diazo ester insertion in N-H bonds of amino acid derivatives.
• sulfonated iron porphyrin is an active catalyst for carbenoid insertion in N–H bonds of aminoacid derivatives in aqueous media.
• insertion is regioselective onto the NH2 termini of chain B of insulin.
Abstract
The metal complex FeTSPPCl (5,10,15,20-tetrakis-(4-sulfonato-phenyl)-porphyrin-iron(III) chloride is an active catalyst for carbenoid insertion in N–H bonds of aminoacid derivatives in aqueous media. A variety of diazoacetates and methyl diazophosphonate were used as carbenoid precursors. The commercially available iron porphyrin complex can also selectively catalyze alkylation of the N- terminus of insulin (chain B).
Keywords
Catalytic NH insertion; iron porphyrin; diazo ester; amino ester; insulin
1. Introduction
The most common bioconjugations are coupling of a small molecule (such as biotin or a fluorescent dye) to a protein. Usual types of bioconjugation chemistry are amine coupling of lysine amino acid residues (typically through amine-reactive succinimidyl esters) and sulfhydryl coupling of cysteine residues (via a sulfhydryl-reactive maleimide) [1]. Generally, the preparation of bifunctional agents involves multistep syntheses, and their subsequent incorporation into biomolecules is often hampered by cross-reactivity or nonspecific interactions with other functional groups present. Thus the quest for novel, efficient strategies for the synthesis of functional agents, and their incorporation into biomolecules, has led to a burgeoning interest in “metallocarbenoid” chemistry [2].
The catalytic N-H insertion of α-diazocarbonyl compounds is a powerful organic reaction which has various useful applications [3-5]. One of the most successful intramolecular N-H insertions has been the conversion of a penicillin analogue into the carbapenem nucleus via rhodium-catalyzed insertion of a keto carbenoid into the N-H bond of the β-lactam [6]. More generally, this insertion reaction can lead to the synthesis of α-amino esters, dipeptides and nitrogen-containing heterocycles. The catalytic preparation of optically active amines, recently reported by Zhou et al.[7]
is also a nice example of application in asymmetric synthesis. Protein modification through alkylation of the N-terminal α-amino acid is also a potentially useful approach such as recently by the group of Che [8]. Among the previously reported metal-catalyzed carbenoid transfer, selective tryptophan modification with rhodium carbenoids [9, 10] and terminal amino acid modification of proteins with ruthenium porphyrins have been previously explored [11], although in the latter case, a stoichiometric amount of the metalloporphyrin was necessary.
We reported the first success in the use of ruthenium porphyrin catalysts for intermolecular insertion of diazo derivatives into N-H bonds in 1997 [12]. Since then, we [13, 14] and others [11,15- 18] have found that ruthenium and iron porphyrins are active catalysts, for carbenoid transfer, not only in organic solvents but also in protic solvents and water. Although, there are examples showing reactions with diazoacetonitrile [19] diazomethyl phosphonate [20] and trifluoro diazomethane[21]
in organic solvents, ethyl diazoacetate has been mainly used as the carbenoid precursors in water.
In this paper, we report NH insertion of amino esters using various diazo esters [5] as reagents and the site selective modification of terminal NH2 group of insulin using water-soluble 5,10,15,20-tetrakis-(4-sulfonato-phenyl)-porphyrin-iron(III) chloride (FeTSPPCl) 1 as catalyst (Scheme 1). Iron porphyrin-catalyzed NH insertions have been previously studied in organic solvents [15-18].
Aviv and Gross [18] also reported intermolecular N-H insertion of anilines in water/THF catalyzed by myoglobin. Although, there are many examples showing that 5,10,15,20-tetrakis-(4-sulfonato- phenyl)-porphyrin-iron(III) chloride is a versatile water-soluble catalyst for oxidations in organic syntheses [22] and few other reactions [23,24], its use for carbenoid transfer in water seems to be neglected [25]. Comparative evaluation of reactivity for N-H insertion of different diazo derivatives with iron and ruthenium porphyrins will also be described.
2. Experimental 2.1. General
All reactions were performed under argon and were magnetically stirred. Solvents were distilled from appropriate drying agent prior to use: MeOH from turning Mg. Commercially available reagents (Acros) were used without further purification unless otherwise stated. All reactions were monitored by TLC with Merck pre-coated aluminium foil sheets (Silica gel 60 with fluorescent indicator UV254). Compounds were visualized with UV light at 254 nm. Column chromatographies were carried out using silica gel from Merck (0.063-0.200 mm). UV-visible spectra were recorded on a UVIKON XL from Biotech. MALDI-TOF mass spectra were recorded on microflex Bruker. The analysis were performed on a ThermoFisher LC-MS system (a DionexUltiMate 3000 quaternary RSLC Nano- UPHLC system coupled with a Q-Exactive Hybrid Quadrupole-Orbitrap Mass Spectrometer). Ethyl diazoacetate and 5,10,15,20-tetrakis-(4-sulfonato-phenyl)-porphyrin-iron(III) chloride are commercially available. 5,10,15,20-tetrakis-(4-sulfonato-phenyl)-porphyrin-ruthenium(II) carbonyl was prepared as previously reported [14]. Diisopropyl diazomethylphosphonate was prepared as previously reported [26,27].
2.2 Syntheses of diazo compounds
The synthesis of substituted benzyl diazoacetates was adapted from the method previously reported by Fukuyama [28]. For the synthesis of the 4-(trifluoromethyl)benzyl 2-diazoacetate, 4- (trifluoromethyl)benzyl bromoacetate was first prepared: 4-(Trifluoromethyl)benzyl alcohol (548 µL, 4.0 mmol) and NaHCO3 (1.0 g, 12.0 mmol) were dissolved in acetonitrile (20 mL) and bromoacetylbromide (524 µL, 6.0 mmol) was added slowly at 0°C in a 100 mL round bottom flask.
After stirring for 15 min at room temperature, the reaction was quenched with H2O. The solution was extracted with CH2Cl2 three times. The organic phase was washed with brine and dried over MgSO4. The solvent was evaporated, and the residue was used in the next reaction without purification. The 4-(trifluoromethyl)benzyl bromoacetate thus obtained and N,N’-ditosylhydrazine (272 mg, 8.0 mmol) were dissolved in THF (20 mL) and cooled to 0°C. DBU (3 mL, 20 mmol) was added dropwise and stirred at room temperature for 10 minutes. After quenching the reaction by the addition of
saturated NaHCO3 solution, this was extracted three times with diethylether. The organic phase was washed with brine, dried over MgSO4 and evaporated to give the crude diazo acetate. Purification of the diazo acetate was performed with neutral silica gel to give 4-(trifluoromethyl)benzyl 2- diazoacetate as a pale yellow oil (897 mg, 90%, two steps). 1H NMR (CDCl3, 400 MHz, ppm): δ 4.83 (s, 1H, CH=N2), 5.25 (s, 2H, CH2), 7.47 (d, J = 8.02 Hz, 2H), 7.63 (d, J = 7.63 Hz, 2H). 13C NMR (CDCl3, 125 MHz, ppm): δ 46.48 (CH=N2), 65.56 (CH2), δ 123.05 (CF3), 125.65 (CHPh), 128.23 (CHPh), 130.57 (Cq-CF3), 140.06 (Cq-CH2), 166.57 (C=O). 19F NMR (CDCl3, 400 MHz, ppm): δ -62.67 (s). HR-MS (m/z):
calculated mass for C10H7N2O2F3Na [M+Na]+: 267.03573, found m/z: 267.0356 (0 ppm).
For the synthesis of the 4-(methylsulfide)benzyl 2-diazoacetate, the previous experimental procedure was employed. The expected product was obtained as a yellow oil (yield 88 %). 1H NMR (CDCl3, 400 MHz, ppm): δ 2.48 (s, 3H, SCH3), 4.79 (s, 1H, CH=N2), 5.16 (s, 2H, CH2), 7.24 – 7.30 (m, 4H, Ph). 13C NMR (CDCl3, 125 MHz, ppm): δ 15.85 (SCH3), 46.48 (CH=N2), 66.28 (CH2), 126.68 (CHPh), 129.07 (CHPh), 132.72 (Cq), 139.08 (Cq), 166.8 (CO2). HR-MS (m/z): calculated mass for C10H10N2O2NaS [M+Na]+: 245.03607, found m/z: 245.0361 (0 ppm).
2.3 General procedure for NH insertion in water
In a typical experiment, the α-amino ester (0.18 mmol) and FeTSPPCl catalyst (1.8 µmol) were placed in a schlenck tube under argon and dissolved in 2 ml of degazed bicarbonate buffer solution (pH 10). Ethyl diazoacetate (0.18 mmol) was then added at room temperature. After 15 min of stirring the reaction was stopped and the mixture was then extracted three times with CH2Cl2, dried by MgSO4 and purified by column chromatography on silica gel (CH2Cl2/Methanol:97/3). A similar procedure was effective for N-H insertion catalyzed by RuTSPPCO, excepted that the reaction was stopped after 4 hours.
2.4 General procedure for NH insertion in organic solvent
In a typical experiment the α-amino ester (0.18 mmol) and FeTPPCl (1.3 mg, 1.8 µmol) were placed in a schlenck tube under argon and dissolved in 2 ml of distilled CH2Cl2. NEt3 (0.27 mmol) was added to the solution. Ethyl diazoacetate) (0.18 mmol) was then added at room temperature. After 2 min of stirring, the insertion product was purified by column chromatography on silica gel (CH2Cl2/Methanol: 95/5).
2.5 NH insertion of Insulin
In a typical experiment, insulin (5 mg, 0.87 µmol) and 0.2 equiv of TPPSFeCl (0.2 mg, 0.17 µmol) were dissolved in 10 ml of PBS and 5 ml of acetonitrile under nitrogen. 0.32 mg of cobaltocene was first added to the solution and then, the diazo derivative (1.3 mmol) diluted in 10 ul of acetonitrile. The solution was stirred for 1 hour. The yield was estimated by MALDI-TOF. After centrifugation, insulin was purified by preparative HPLC and then dried under vacuum to give a white powder.
2.6 Characterization of N-H inserted compounds
2.6.1 Reaction of ethyl diazoacetate with amino esters
Mono-inserted Tyrosine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.23 (t, J = 7.2 Hz, 3 H, CH3), 2.88-2.98 (m, , 2H, CH2), 3.37 (sys AB, J = 17 Hz, 2 H, CH2), 3.57 (t, 6.8 Hz, 1H, CH), 3.67 (s, 3 H, OCH3), 4.14 (q, 7 Hz ,2 H, CH2), 6.71 (d, J = 8.5 Hz, 2 H, CH), 7.02 (d, 8.5 Hz, 2 H, CH).13C NMR (125 MHz, CDCl3, ppm): δ 14.13 (CH3 (CO2Et)), 38.56 (CH2Ph), 49.11 (CH2NH), 51.92 (CH3 (CO2Me)), 61.02 (CH2
(CO2Et)), 62.23 (*CH), 115.51 (CH, Ph), 128.33 (Cq, CH2Ph), 130.32 (CH, Ph), 154.9 (Cq (PhOH)), 171.75 (Cq (CO2Et)), 174.22 (Cq (CO2Me)). HR-MS (m/z): calculated mass for C14H19NO5 [M+Na]+ : 304.29,, found m/z: 304.11(3 ppm)
Bis-inserted Tyrosine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.25 (t, J = 7.2 Hz, 6H, 2xCH3), δ 2.91-2.99 (m, 2H, CH2), δ 3.57 (s, 3H, OCH3), 3.63 (t, 6.8 Hz, 1H, CH), 3.71 (s, 4 H, 2xCH2), δ 4.14 (q, 7 Hz, 2H, CH2), δ 6.71 (d, J = 8.5 Hz, 2H, CH), δ 7.02 (d, 8.5 Hz, 2H, CH). 13C NMR (125 MHz, CDCl3, ppm):
δ 14.32 (CH3 (CO2Et)), δ 36.37 (CH2Ph), δ 51.50 (CH3 (CO2Me)), δ 52.85 (CH2N), δ 60.82 (CH2 (CO2Et)), δ 67.12 (*CH), δ 115.38 (CH, Ph), δ 129.34 (Cq, CH2Ph), δ 130.48 (CH, Ph), δ 154.56 (Cq (PhOH)), δ 171.60 (Cq (CO2Et)), δ 172.47 (Cq (CO2Me). HR-MS (m/z): calculated mass for C18H25NO7Na [M+Na]+: 390.15287, found m/z: 390.1528 (0 ppm).
Mono-inserted tryptophan methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.19 (t, J = 7.1 Hz, 6H, CH3), 3.12-3,27 (m, 2H, CH2), 3.39 (sys AB, J = 17 Hz, 2H, CH2), 3.66 (s, 3 H, OCH3), 3.70 (t, 6.1 Hz, 1H, CH), 4.10 (q, 7.1 Hz, 2H, CH2), 7.08-7.13 (m, 2H, CH), 7.18 (t, J = 7.3 Hz, 1H, CH), 7.34 (d, J = 8 Hz, 1H, CH), 7.6 (d, J = 7.8 Hz, 1H, CH), δ 8.19 (s, 1H, NH (indole)). 13C NMR (125 MHz, CDCl3, ppm): δ 14.25 (CH3(CO2Et)), 29.27 (CH2Ph), 49.40 (CH2N), 52.04 (CH3 (CO2Me)), 60.97 (CH2 (CO2Et)), 61.32 (*CH), 111.03 (Cq), 111.32 (CH), 118.83 (CH), 119.63 (CH), 122.27 (CH), 123.08 (CH), 127.54 (Cq), 136.35 (Cq), 171.78 (Cq (CO2Et)), 174.52 (Cq (CO2Me)). HR-MS (m/z): calculated mass for C16H20N2O4Na [M+Na]+: 327.13208; found m/z: 327.1322.
Bis-inserted tryptophan methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.25 (t, J = 7.1 Hz, 6H, 2xCH3), δ 3.13-3.28 (m, 2H, CH2), δ 3.57 (s, 3H, OCH3), 3.76 (s, 4H, 2xCH2), 3.82 (t, 7.4 Hz ,1H, CH), δ 4.14 (q, J = 7 Hz, 2H, CH2), δ 7.11 (t, J = 7.3 Hz, 1H, CH), 7.17 (t, J = 7.2 Hz, 2H, CH), 7.33 (d, J = 8 Hz, 1H, CH), 7.63(d, J = 7.8 Hz, 1H, CH), δ 7.99 (s, 1H, NH (indole)). 13C NMR (125 MHz, CDCl3, ppm): δ 14.34 (CH3 (CO2Et)), δ 26.58 (CH2Ph), δ 51.45 (CH3 (CO2Me)), δ 52.78 (CH2N), δ 60.73 (CH2 (CO2Et)), δ 65.62 (*CH), δ 111.16 (CH), δ 111.45 (Cq), δ 118.79 (CH), δ 119.51 (CH), δ 122.06 (CH), δ 123.15 (CH), δ 127.61 (Cq), δ 136.15 (Cq), δ 171.61 (Cq (CO2Et)), δ 172.76 (Cq (CO2Me). HR-MS (m/z): calculated mass for C20H26N2O6Na [M+Na]+: 413.16886; found m/z: 413.1701.
Mono-inserted phenylalanine methyl ester : 1H NMR (500 MHz, CDCl3, ppm): δ 1.23 (t, J = 7.2 Hz, 3H, CH3), 2.94-3.05 (m, 2H, CH2), 3.36 (sys AB, J = 17 Hz, 2H, CH2NH), 3.60 (t, J = 6.7 Hz ,1H, CH), 3.66 (s, 3H, CH3), δ 4.14 (q, J = 7.1 Hz, 2H, CH2), δ 7.19-7.30 (m, 5H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 14.13 (CH3 (CO2Et)), 39.67 (CH2Ph), 49.32 (CH2NH), 51.85 (CH3 (CO2Me)), 60.99 (CH2 (CO2Et)), 62.31 (*CH), 127 (CH, Ph), 128.65 (CH, Ph), 129.32 (CH, Ph), 137.04 (Cq (PhCH2)), 171.72 (Cq (CO2Et)), 174.21 (Cq (CO2Me). HR-MS (m/z): calculated mass for C14H19NO5Na [M+Na]+: 288,12118; found m/z:
288,1207 (2 ppm).
Bis-inserted phenylalanine methyl ester : 1H NMR (500 MHz, CDCl3, ppm): δ 1.25 (t, J = 7.1 Hz, 6H, 2xCH3), 2.99-3.08 (m, 2H, CH2), 3.57 (s, 3H, CH3), 3.66-3.70 (m, 1H, CH), 3.72 (s, 4H, 2xCH2), 4.14 (q, J
= 7.1 Hz, 4H, 2xCH2), 7.17-7.27 (m, 5H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 14.34 (CH3 (CO2Et)), 37.28 (CH2Ph), 52.87 (CH2NH), 51.46 (CH3 (CO2Me)), 60.75 (CH2 (CO2Et)), 66.9 (*CH), 126.69 (CH, Ph),
128.50 (CH, Ph), 129.37 (CH, Ph), 137.50 (Cq (PhCH2)), 171.48 (Cq (CO2Et)), 172.26 (Cq (CO2Me)). HR- MS (m/z): calculated mass for C18H25NO6Na [M+Na]+: 374.15796; found m/z: 374.1590 (3 ppm).
Mono-inserted glycine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.25 (t, J = 7.13 Hz, 3H, CH3
(CO2Et), 3.44 (s, 2H, CH2), 3.46 (s, 2H, CH2 (CH2NH)), 3.71 (s, 3H, CH3 (CO2Me)), 4.17 (q, J = 7.17 Hz, 2H, CH2 (CO2Et)). 13C NMR (125 MHz, CDCl3, ppm): δ 14.27 (CH3 (CO2Et)), 50.05 (CH2), 50.23 (CH2), 51.93 (CH3), 60.95 (CH2 (CO2Et)), 171.83 (Cq (CO2Et)), 172.30 (Cq (CO2Me)). HR-MS (m/z): calculated mass for C7H13NO4Na [M+Na]+: 198,07423; found m/z: 198,0748 (3 ppm).
Bis-inserted glycine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.24 (t, J = 7.2 Hz, 3H, CH3
(CO2Et), 3.63 (s, 4H, CH2(NHCH2)), 3.65 (s, 2H, CH2(CH2NH)), 3.69 (s, 3H, CH3(CO2Me)), 4.15 (q, J = 7.2 Hz, 2H, CH2(CO2Et)). 13C NMR (125 MHz, CDCl3, ppm): δ 14.31 (CH3 (CO2Et)), 51.81 (CH3 (CO2Me)), 55.10 (CH2 (CH2CO2Me)), 55.29 (NHCH2), 60.80 (CH2 (CO2Et)), 170.87 (Cq (CO2Et)), 171.37 (Cq (CO2Me)). HR-MS (m/z): calculated mass for C11H19NO6Na [M+Na]+: 284.11101; found m/z: 284.11111 (0 ppm).
Mono-inserted lysine methyl ester: 1H NMR (500MHz, CDCl3, ppm): δ 1.23 (t, 3H, CH3), 1.45 (m, 4H, 2xCH2), 1.65 (m, 2H, CH2), 2.69 (t, 2H, CH2), 3.36 (sys AB, 17.0 Hz, 2H, CH2NH), 3.71 (s, 3H, CH3), 4.15 (q, 2H, CH2), 4.17 (m, 1H, CH).
Mono-inserted S-H cysteine ethyl ester: 1H NMR (400MHz, CDCl3, ppm): δ 1.32 (2t, 6H, CH3), 2.97- 3.13 (m, 2H, CH2C*), 3.26 (s, 2H, CH2S), 3.79 (m, 1H, C*H), 4.24 (2m, 4H, CH2) containing also 20% of N-H inserted cysteine.
2.6.2 Reaction of 4-(trifluoromethyl)benzyl 2-diazoacetate with amino esters
Mono-inserted phenylalanine methyl ester: 1H NMR (500MHz, CDCl3, ppm): δ 2.93–3.05 (m, 2H, CH2), 3.42 (sys AB, J = 17.40 Hz, 2H, CH2NH), 3.61 (t, J = 6.90 Hz, 1H, *CH), 3.65 (s, 3H, OCH3), 5.16 (s, 2H, OCH2), 7.17–7.29 (m, 4H, Ph), 7.42 (d, J = 8 Hz, 2H, Ph), 7.60 (d, J = 8 Hz, 2H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 39.67 (CH2), 49.27 (CH2), δ 51.99 (CH3O), 62.23 (*CH), 65.71 (CH2), 124.09 (q,
1JCF = 272.14 Hz, CF3), 125.68 (q, 3JCF = 3,78 Hz, Ph), 127.04 (Ph), 128.40 (Ph), 128.66 (Ph), 129.31 (Ph), 130.66 (q, 2JCF = 32.57 Hz, Cq), 136.95 (Cq), 139.59 (Cq), 171.45 (Cq, CO2CH2), 174.13 (Cq, CO2Me). NMR 19F (CDCl3, 376 MHz, ppm): δ -62.685.HR-MS (m/z): calculated mass for C20H20NO4F3Na [M+Na]+: 418,12421; found m/z: 418,1239 (1 ppm).
Mono-inserted tryptophan methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 2.07 (s, 1H, NH), δ 3.12–
3.28 (m, 2H, CH2), 3.48 (sys AB, J = 17.38 Hz, 2H, CH2-NH), 3.66 (s, 3H, CH3), 3.72 (t, J = 7.08 Hz, 1H, CH), 5.12 (sys AB, J = 6.82 Hz, 2H, CH2Ph), 7.07 (d, J = 1.99 Hz, 1H, Hindole), 7.13 (t, J = 7.75 Hz, 1H, Hindole), 7.2 (t, J = 7.69 Hz, 1H, Hindole), 7.34 (d, J = 8.11 Hz, 1H, Hindole), 7.38 (d, J = 8.01 Hz, 2H, Ph), 7.59 (d, J = 7.97 MHz, 2H, Ph), 7.60 (d, J = 7.04 Hz, 1H, Hindole), δ 8.12 (s, 1H, NHindole). 13C NMR (125 MHz, CDCl3, ppm): δ 29.27 (CH2), 49.34 (CH2), 52.05 (OCH3), 61.26 (*CH), 65.63 (CH2), 110.86 (Cq), 111.37 (CHNH), 118.75 (Ph), 119.59 (Ph), 122.24 (Ph), 123.15 (Ph), 124.08 (q, 1JCF = 272.04 Hz, CF3), 125.61 (q, 3JCF = 3.74 Hz, CH), 127.47 (Cq), δ 128.33 (Ph), 130.54 (q, 2JCF = 32.42, Cq), 136.36 (Cq), 139.57 (Cq), 171.54 (Cq, CO2CH2), 174.48 (Cq, CO2Me). 19F NMR (CDCl3, 376 MHz, ppm):
δ -62.632. HR-MS (m/z): calculated mass for C22H21N2O4F3Na [M+Na]+: 457.1351; found m/z: 457.135 (0 ppm).
Mono-inserted tyrosine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 2.87–2.96 (m, 2H, CH2), 3.46 (sys AB, J = 17.4 Hz, 2H, CH2NH), 3.59 (t, J = 6.60 Hz, 1H, CH), 3.67 (s, 3H, CH3), δ 5.16 (s, 2H, CH2OCO), 6.69 (d, J = 8.44, Hz, 2H, PhOH), 7.0 (d, J = 8.44 Hz, 2H PhOH), ), 7.4 (d, J = 8.03 Hz, 2H PhCF3), 7.6 (d, J = 8.1 Hz, 2H PhCF3).13C NMR (125 MHz, CDCl3, ppm): δ 38.59 (CH2), 49.11 (CH2), 52.1 (CH3O), 62.23 (*CH), 65.84 (CH2), 115.68 (Ph), 124.05 (q, 1JCF = 272.08 Hz, CF3), 125.65 (q, 3JCF = 3.71 Hz, CH), 128.12 (Cq), 128.37 (Ph), 130.40 (Ph), 130.45 (q, 2JCF = 32.51 Hz, Cq), 139.42 (Cq), 155.15 (Cq), 171.56 (Cq, CO2CH2), 174.32 (Cq, CO2Me). 19F NMR (CDCl3, 376 MHz, ppm): δ -62.668. HR-MS (m/z): calculated mass for C20H20NO5F3Na [M+Na]+: 434.11913; found m/z: 434.1187 (1 ppm).
Mono-inserted phenylalanine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 4.00 (s, 2H, CH2, (NHCH2) ), δ 5.26 (s, 2H, CH2), 6.62 (d, J = 7.71 Hz, 2H, o-Ph ), δ 6.77 (t, J = 7.34 Hz, 1H, p-Ph), 7.20 (t, 2H, J = 7.44 Hz, m-Ph), 7.42 (d, J = 8.01 Hz, 2H, Ph), 7.62 (d, J = 8.01 Hz, 2H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 46.02 (CH2), 66.10 (CH2), 113.19 (CH), 118.70 (CH), 124.07 (q, 1JCF = 272.17 Hz, CF3), 125.72 (q, 2JCF = 3.72 Hz, CH), 128.41 (Ph), 129.50 (Ph), 130.73 (q, 2JCF = 32.53 Hz, Cq), 139.41 (Cq), 146.99 (Cq), 171.12 (Cq, CO2CH2). NMR 19F (CDCl3, 376 MHz, ppm): δ -62.682. HR-MS (m/z):
calculated mass for C16H14NO2F3Na [M+Na]+: 332.08743; found m/z: 332.0873 (0 ppm).
2.6.3 Reaction of 4-(methylsulfide)benzyl 2-diazoacetate with amino acid derivatives
Mono-inserted aniline: 1H NMR (500 MHz, CDCl3, ppm): δ 2.38 (s, 3H, (SCH3)), 3.83 (s, 2H, (CH2) NHCH2 ), 5.06 (s, 2H, CH2Ph), 6.50 (d, J = 7.69 Hz, 2H,o-Ph ), 6.65 (t, J = 7.34 Hz, 1H, p-Ph), 7.08 (t, 2H, J = 7.42 Hz, o-Ph), 7.15 (sys AB, J = 8.65 Hz, 4H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 15.71 (SCH3), 45.98 (CH2), 66.76 (CH2), 113.11 (CH), 118.37 (CH), 126.57 (CH), 129.16 (CH), 129.40 (CH), 132.05 (Cq), 139.31 (Cq), 147.03 (Cq), 171.15 (Cq). HR-MS (m/z): calculated mass for C16H17NO2NaS [M+Na]+: 310.08777; found m/z: 310.0879 (0 ppm).
Mono-inserted phenylalanine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 2.01 (s, 1H, NH), 2.5 (s, 3H, CH3S), 2.96–3.08 (m, 2H, CH2), 3.43 (sys AB, J = 16.34 Hz, 2H, CH2NH), 3.64 (t, J = 6.81 Hz, 1H, CH), 3.67 (s, 3H, CH3O), 5.1 (s, 2H, CH2O), 7.2–7.31 (m, 9H, Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 15.78 (CH3S), 39.57 (CH2), 49.24 (CH2), 51.89 (CH3O), 62.17 (*CH), 66.32 (CH2), 123.90 (Ph), 126.60 (Ph), 126.94 (Ph), 128.58 (Ph), 129.120 (Ph), 129.26 (Ph), 132.26 (Cq), 136.94 (Cq), 139.13 (Cq), 171.52 (Cq, CO2CH2), 174.08 (Cq, CO2Me). HR-MS (m/z): calculated mass for C20H23NO4NaS [M+Na]+: [M+Na]+:
396.12455; found m/z: 396.1248 (1 ppm).
Mono-inserted tryptophan methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.93 (s, 1H, NH), 2.47 (s, 3H, CH3S), 3.15–3,27 (m, 2H, CH2), 3.44 (sys AB, J = 16.35 Hz, 2H, CH2NH), 3.64 (s, 3H, CH3O), 3.69 (t, J
= 6.08 Hz, 1H, CH), 5.03 (sys AB, J = 5.12 Hz, 2H, CH2Ph), 7.09 (d, J = 2.02 Hz, 1H, Hindole), 7.12 (t, J = 7.78 Hz, 1H, Hindole), 7.18 (t, J = 7.29 Hz, 1H, Hindole), 7.35 (d, J = 8.03 Hz, 1H, Hindole), 7.6 (d, J = 7.84 Hz, 1H, Hindole), 8.07 (s, 1H, NHindole). 13C NMR (125 MHz, CDCl3, ppm): δ 15.73 (CH3S), 29.21 (CH2), 49.35 (CH2), 52.0 (CH3O), 61.23 (*CH), 66.29 (CH2), 110.71 (Cq), 111.35 (CH), 118.69 (CH), 119.48 (CH), 122.11 (CH), 123.19 (CH), 126.53 (CH), 127.43 (Cq), 129.1 (CH), 132.23 (Cq), 136.3 (Cq), 139.06 (Cq), 171.66 (Cq, COOCH2), 174.48 (Cq, CO2Me). HR-MS (m/z): calculated mass for C22H24N2O4NaS [M+Na]+: 435.13545; found m/z: 435.1357 (1 ppm).
Mono-inserted tyrosine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 2.51 (s, 3H, CH3S), 2.9–3.02 (m, 2H, CH2), 3.45 (AB sys, J = 16.48 Hz, 2H, CH2NH), 3.61 (t, J = 6.72 Hz, 1H, CH), 3.7 (s, 3H, CH3O), 5.10 (s, 2H, CH2), 6.72 (d, J = 8.37 Hz, 2H, PhOH), δ 7.04 (d, J = 8.33 Hz, 2H, PhOH), δ 7.24–7.28 (m,
4H, PhSMe). 13C NMR (125 MHz, CDCl3, ppm): δ 15.73 (CH3S), 38.59 (CH2), 49.17 (CH2), 52.05 (CH3O), 62.24 (C*H), 66.51 (CH2), 115.65 (Ph), 126.57 (Ph), 128.13 (Cq), 129.14 (Ph), 130.37 (Ph), 132.1 (Cq), 139.14 (Cq), δ 155.13 (Cq), 171.68 (Cq, CO2CH2), 174.3 (Cq, COOMe). HR-MS (m/z): calculated mass for C20H23NO5NaS [M+Na]+: [412.11946; found m/z: 412.1195 (0 ppm).
2.6.4 Reaction of diisopropyl diazomethylphosphonate with amino acid derivatives
Mono-inserted aniline: 1H NMR (500 MHz, CDCl3, ppm): δ 1.32 (d, J = 6.2 Hz, 6H, 2xCH3), 1.37 (d, J = 6.2 Hz, 6H, 2xCH3), 3.46 (d, JHP = 13.7 Hz, 2H, CH2), 3.96 (s broad, 1H, NH), 4.73-4.82 (m, JHH = JHP = 6.3 Hz, 2H, CHOP), 6.68 (d, J = 7.8 Hz, 2H, Ph), 6.77 (t, J = 7.3 Hz, 1H, Ph), 7.21 (t, J = 7.5 Hz, 2H, Ph).
13C NMR, HMBC (125MHz, CDCl3, ppm): δ 23.94 (d, 3JCP = 4.7 Hz, 2xCH3), 24.06 (d, 3JCP = 3.8 Hz, 2xCH3), 41.03 (d, 1JCP = 156.13 Hz, NHCH2), 71.18 (d, 2JCP = 6.72 Hz, 2xCHOP), 113.26 (o-Ph), 118.37 (p- Ph), 129.19 (m-Ph), 147.67 (d, 3JCP = 3.8 Hz, Cq). 31P NMR (162 MHz, CDCl3, ppm): δ 22.68 (s). HRMS (m/z): calculated mass for C13H22NO3NaP (M+Na)+: 294.1235; found m/z: 294.1236 (0 ppm).
Mono-inserted tryptophan methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.22 (t, J = 6.44 Hz, 6H, 2xCH3), δ 1.26 (t, J = 6.59 Hz, 6H, 2xCH3), δ 2.79 (t, J = 14.37 Hz, 1H, CH2), 3.04 (t, J = 12.94 Hz, 1H, CH2), 3.08-3.21 (m, 2H, CH2 (tryp)), 3.63 (s, 3H, CH3 (OCH3)), 3.73 (t, J = 6.39 Hz, 1H, *CH), 4.6-4.7 (m, JHH = JHP = 6.3 Hz, 2H, CHOP), 6.99 (s, 1H, Hindole), 7.08 (t, 1H, 7.38 Hz, Ph), 7.15 (t, 1H, J = 7.76 Hz, Ph), 7.33 (d, 1H, J = 8.1 Hz, Ph), 7.57 (d, 1H, 7.88 Hz, Ph).13C NMR (125 MHz, CDCl3, ppm): δ 23.96 (2d, J = 5.09, 5.78 Hz, 2xCH3), 24.12 (d, J = 3.74 Hz, 2xCH3), 29.13 (CH2, (tryp)), 44.47 (d, 1JCP = 158.17 Hz, NHCH2), 51.86 (OCH3), 62.82 (d, 3JCP = 15.44 Hz, *CH), 70.78 (d, 2J = 1.8 Hz, CH), 70.94 (s, CH), 110.68 (Cq indole), 111.38 (CH indole), 118.71 (CH indole), 119.35 (CH indole), 121.95 (CH indole), 123.20 (CH indole), 127.54 (Cq indole), 136.39 (Cq indole), 174.55 (Cq (CO)).31P NMR (162MHz, CDCl3, ppm):
δ 23.13 (s). HR-MS (m/z): calculated mass for (C19H29N2O5NaP) [M+Na]+: 419.1711; found: 419.1712 (0 ppm).
Mono-inserted tyrosine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.22 (t, J = 5.81 Hz, 6H, CH3), 1.26 (d, J = 6.18 Hz, 6H, CH3), 2.74 (t, J = 14.38 Hz, 1H, CH2), 2.77-2.93 (m, 2H, CH2 (tyrosine)), 3.01 (t, J = 11.85 Hz, 1H, CH2), 3.59 (t, J = 6.58 Hz, 1H, *CH), 3.65 (s, 3H, OCH3), 4.58-4.69 (m, JHP = JCP = 6.01 Hz, 2H, CHOP), 6.74 (d, 2H, 8.33 Hz, Ph), 6.96 (d, 1H, 8.33 Hz, CH (Ph). 13C NMR (125 MHz, CDCl3, ppm): δ 23.96 (2d, 3JCP = 4.86 Hz, 2xCH3), δ 24.1 (d, 3JCP = 3.63 Hz, CH3), 38.67 (CH2 (tyrosine)), 44.32 (d, 1JCP = 160.04 Hz, NHCH2), 51.86 (OCH3)), 63.71 (d, 3JCP = 14.96 Hz, *CH), 71.42 (2d, 2JCP = 6.84 Hz, CH) 115.58 (Ph), 127.54 (Cq, Ph), 130.24 (Ph), 156.1 (Cq), 174.41 (Cq (CO2Me)). 31P NMR (162 MHz, CDCl3, ppm): δ 22.93 (s). HR-MS (m/z): calculated mass for (C17H28NO6NaP) [M+Na]+: 396.1552;
found: 396.1553 (0 ppm).
Mono-inserted phenylalanine methyl ester: 1H NMR (500 MHz, CDCl3, ppm): δ 1.24 (t, J = 7.16 Hz, 6H, 2xCH3), 1.27-1.29 (m, 6H, 2xCH3), 1.71 (s large, 1H, NH), 2.76 (t, 1H, CH2 (NHCH2)), 2.89-3.02 (m, CH2
(Phenylala.) + 1 CH2 (NHCH2)), 3.64 (t, 6.77 Hz, 1H, *CH), 3.67 (s, 3H, (OCH3)), 4.61-4.70 (m, 2H, CH, 7.17-7.28 (m, 5H, Ph)). 13C NMR (125 MHz, CDCl3, ppm): δ 24.06 (2d, 3JCP = 4.9 Hz, 2xCH3(isopropyl)), 24.2 (2d, 3JCP = 2.57 Hz, 2xCH3), 39.56 (CH2 (phenylala.)), 45.29 (CH2, NHCH2), 51.88 (OCH3)), 63.66 (d,
3JCP = 15.42 Hz, *CH), 70.91 (2d, 2JCP = 6.52, 6.93Hz, 2xCH, 126.88 (Ph), 128.56 (Ph), 129.36 (Ph), 137.23 (Cq), 174.28 (Cq (CO2Me)). NMR 31P (162 MHz, CDCl3, ppm): 22.73 (s). HR-MS (m/z):
calculated mass for C17H28NO5PNa [M+Na]+: 380.16028; found m/z: 380.1598 (1 ppm).
3. Results and discussion
3.1 Synthesis of diazo derivatives
Besides ethyl diazoacetate and diisopropyl diazomethylphosphonate, two substituted benzyl diazo acetates were also investigated in order to introduce SMe and trifluoromethyl groups. A tosyl group was employed for the synthesis due the availability of inexpensive p-toluenesulfonyl hydrazide. The first step was the preparation of 4-(trifluoromethyl)benzylbromoacetate and 4- (methylsulfide)benzylbromoacetate from 4-(trifluoromethyl)benzyl alcohol and 4- (methylsulfide)benzyl alcohol, respectively. Finally, these bromoacetates underwent an easy conversion to the corresponding diazo acetates upon treatment with N,N’-ditosylhydrazine and DBU at 0°C, following the protocol previously reported by Fukuyama[28]. The preparation of 4- (trifluoromethyl)benzyl 2-diazoacetate proceeded in a 90% yield, while that of 4- (methylsulfide)benzyl 2-diazoacetate in 88% yield. These diazoacetates were characterized by 1H and the latter by 19F NMR spectroscopy. We note the presence of a characteristic peak at 4.83 ppm and 4.79 ppm (broad singlet) corresponding to CH neighboring N2 for the CF3 and SCH3 derivatives, respectively.
3.2 NH insertion of amino acid esters by ethyl diazoacetate in water.
The insertion of electrophilic carbenes into the N-H bonds of α-amino acids and α-amino acid esters could be a powerful method for N-alkylating this important class of compounds. However there are several drawbacks and the regioselectivity of the reaction still appears as a difficult problem to solve. This is due to numerous possible reactive sites such as acidic groups, amino groups, thiol groups and phenol groups. To shine some light on this aspect, we first investigated the regioselectivity of the reaction catalyzed by 5,10,15,20-tetrakis-(4-sulfonato-phenyl)-porphyrin- iron(III) chloride (FeTSPPCl) (Figure 1) in water.
The results are summarized in Table 1. Thus FeTSPPCl catalyzed decomposition of 1 equiv of ethyl diazoacetate in the presence of tyrosine methyl ester in dichloromethane resulted in the formation of mono N-alkylated amino esters in 71% yield, containing 14% of bis-inserted compound (Table 1, entry 4). A similar result was obtained for tryptophan with 66% yield (Table 1, entry 3). In this case 18% of insertion was the bis-inserted compound (Scheme 2). To extend the scope of this N- H insertion, the reaction of a number of amino acid derivatives with ethyl diazoacetate in the presence of TSPPFeCl as catalyst was studied at room temperature (25°C). In all these experiments, N-H insertion is the major reaction yielding alkylated products with moderate yield (43-71%), usually obtained with a small amount of bis-inserted compounds. Glycine is an exception since, in this case, the bis-inserted compounds was obtained with 32% yield (Table 1, entry 6). A complete regioselectivity is obtained with tyrosine, since no O-H insertion was detected. With tryptophan methyl ester, only α−amino group was reactive since no insertion was detected in the NH group of indole ring. Because the difference of pKa values of the N-terminal group (~7.6-8) and the NH2 group in the side chain of lysine residue (10.0-10.2), this reaction (81% yield) can also be optimized to maximize the selectivity for the N-terminal insertion by the reaction being performed under weakly basic conditions (pH 9). Actually cysteine ethyl ester was also tested to detect any selectively. With this amino ester, both N-H insertion and S-H insertion were observed, the latter being largely predominant (> 70%, from 1H NMR) using one equivalent of the diazo derivative. Unfortunately, we were not able to separate the two compounds.
The diazo acetates were added to the amino acids in a water-methanol solution containing the catalyst under argon. Argon was used as a protecting atmosphere due to a possible formation of reduced iron (II) in the catalytic cycle [17]. The reactions were followed by TLC and were left until the complete disappearance of the diazo acetates. We note that traces of the amino acids remained when the reactions were over and this is due to the formation of carbene dimers. The products were characterized by NMR spectroscopy. We note in the 1H NMR spectrum, the presence of characteristic AB systems (~ 3.4 ppm), which are due to the two protons on the carbon next to the amine function.
For example, the spectrum resulting after diazo insertion in tryptophan is shown Figure 2. This result was observed for all chiral amino esters and is related to the presence of the asymmetric carbon atom. With glycine, only one singlet was noted at 3.6 ppm.
Although there are a large range of strategies and reagents for coupling amino acids available, racemization of amino acids is still a major problem in these reactions [29]. Due to the basic pH of the carbonate buffer solution, it was necessary to verify the absence of racemization during the insertion process [29, 30]. Phenylalanine methyl ester was used as substrate to check a possible racemization. After ethyl diazoacetate coupling, the enantiomeric and the racemic forms of phenylalanine were analyzed separately on HPLC column with chiral phase as the stationary phase.
The results are summarized in Figure 3. The chiral analysis gave 50% of each isomer after insertion with racemic phenylalanine whereas a pure enantiomer was obtained with L-phenylalanine, as expected for a reaction without any racemization.
3.3 NH insertion of amino acid esters by benzyl diazocetate derivatives
We first investigated the reaction of 4-(thiomethyl)benzyl diazoacetate and 4- (trifluoromethyl)benzyl diazoacetate with aniline and amino esters (Table 2) in organic solvent. In particular, we wanted to check if these reactions can tolerate the presence of sulfur atom in the diazo compound. The insertion reactions are shown in Scheme 3 and the results reported in Table 2.
In these case, the catalyst used for these reaction was TPPFeCl (Figure 1). The reactions are very fast (< 2 mn). The yields obtained are quite high (87-88%) with the diazoester bearing the CF3 groups if we except the results obtained with tyrosine (66%)(Table 2, entry 4) due probably to solubility difficulty in CH2Cl2. Aniline gave also a good yield (97%) (Table 2, entry 1). In contrast, the yields are only moderate with the diazo bearing the SMe group (50-67%) and some amount of the starting material was recovered (Table 2, entries 5-8). A possible interaction of the sulfur atom with iron may inhibit the catalyst, and consequently may explain the low conversion. As previously indicated with ethyl diazoacetate, all the insertion products were characterized by NMR, showing also an AB system for CH2 group in alpha position of the nitrogen.
To examine whether these diazo derivatives could be used for intermolecular N-H insertion of amino acid derivatives in protein, the catalytic reaction was also realized with water soluble tetra- p-(SO3-)phenyl metalloporphyrins (Fe and Ru). These two metalloporphyrins were chosen because they are easily prepared and commercially available. With the goal of developing protein-compatible reaction conditions, we selected methanol as a co-solvent for diazo ester and amino acid derivative solubilities. All the reactions with benzyl diazoacetate derivatives are summarized in Table 3. First, it should be noted that the yields are only moderare (40-60%) using a small excess of the diazo derivatives (2 equiv) vs substrate. In these water conditions, insertion of the α-amino group was only observed with lysine methyl ester. As both ruthenium and iron porphyrins react with nitrogen
compounds to form stable adducts, their decreased activity (by comparison with cyclopropanation reactions)[31] can be attributed to competitive coordination between the amino substrate and ethyl diazoacetate onto the metal. Since axial Ru-N bond seems much stronger than Fe-N bond, iron catalysts are more efficient but less tolerant to functional group such as thiol groups.
In order to give a clearer characterization of the products, a HMBC spectrum was done for the NH insertion reaction of tyrosine with 4-(methylsulfide)benzyl 2-diazoacetate (Figure 3). This spectrum shows the coupling between a carbon and hydrogen present on the neighboring carbon.
We note a coupling between the protons of CH2 (CH2NH) and the asymmetric carbon of the amino acid. Also, these protons are coupled to the carbon of the ester function. This result proves the formation of the NH-CH2 bond.
3.4 NH insertion of amino acid esters by diisopropyl diazomethylphosphonate
Intramolecular N-H insertion of diazomethylphosphonate derivatives was first developed for the synthesis of antibiotic carbapenems [32]. Since then Moody et al [33] have developed significant diazomethylphosphonate-insertion reactions using rhodium complexes as catalysts for a carbenoid approach to peptide synthesis [34]. Many other biological applications have been related to the use of N-(phophomethyl) aminoacids such as shikimate pathway inhibitors [35] and selective thrombin inhibitors [36].
We previously reported the catalytic effectiveness of ruthenium porphyrins for carbenoid generation in reactions of diisopropyl diazomethylphosphonate with alkenes (cyclopropanation) [37]
and allylic amine (N-H insertion) [38]. We herein report the N-H insertion of diazophosphonate into amino acid derivatives catalyzed by FeTSPPCl. The results are summarized in Table 4. In a typical reaction, catalytic insertions into N-H bonds were run in in water/methanol solution at room temperature under argon atmosphere with a substrate:diazo:catalyst ratio of 100:100:1. As shown in Table 4, the coupling products are formed with good yield 81-96% both for aniline and amino acid derivatives without detection of dimerization. Again, no OH insertion was detected for tyrosine.
3.5 NH insertion of insulin by diazo derivatives in water-acetonitrile mixture
Insulin is a polypeptide hormone that is critical for the metabolism of glucose. The insulin monomer consists of two chains, a 21-residue A chain and a 30-residue B chain, linked by a pair of disulfide bonds, A7-B7 and A20-B-19; an additional intrachain disulfide bond links A6 and A11. While insulin is synthesized and stored in the pancreas as a hexamer, the hormonally active form is the monomer. Because the hormone contains many functional different groups, it is a good candidate to test our catalytic system for bioconjugation.
Coupling was first attempted in the pH range 7.4 using ethyl diazoacetate and TSPPFeCl as catalyst. Cobaltocene was used in the insertion of insulin to reduce iron(III) to iron(II). In the previous experiments, ligation of the amino group of the amino acid derivatives to iron is supposed to facilitate the reaction [17]. This is unlikely with insulin. In these experimental conditions, reaction of
ethyl diazoacetate with insulin in PBS/acetonitrile (7/4) at 25°c for 1h afforded the coupling product with 90% insulin conversion containing 10% of double insertion (see Figure 4 and Table 5).
Purification was carried out by dialysis to remove reagents of molecular weight <3000 Da followed by HPLC chromatography to remove residual unconjugated insulin.
Analysis by MALDI-TOF MS upon purification by HPLC, revealed a peak at 5734 + 86 = 5820 (MH+), consistent with the incorporation of one acetate carbene unit. Electrospray MS/MS (see experimental section and Figure 5) of this mono-ligated species as compared to unmodified insulin, revealed attachment of the ester moiety to the FVNQHL peptide only, corresponding to the N termini of chain B. Fragmentation of this peptide yielded peaks at m/z = 206 (ion a1), m/z = 305 (ion a2), m/z
= 333 (ion b2), m/z = 447 (ion b3), m/z = 676 (ion b4) and m/z = 712 (ion b5), according to the modification of the phenylalanine in N-termini. In contrast, fragmentation of chain A, revealed two unmodified peptides, one from the N-termini (GIVEQ) and the second to the acid termini (GERGFFYTPKA). This result showed that neither N termini nor the NH2 of lysine was tethered by the ester group of the diazo precursor.
In order to confirm this result, 1H NMR analysis of modified insulin revealed an AB spectrum for the CH2 group, in alpha position of NH, located at 3.4 ppm as expected for a chiral amino acid. In contrast the CH2 group ligated to Gly, revealed a singlet according to the results obtained with the amino ester models. Moreover, the methyl group, of the ligated acetate can also be detected in the NMR spectrum (Figure 6).
Other diazo derivatives were also tested for coupling to insulin (see Table 5). Whereas 4- (methylsulfide)benzyl 2-diazoacetate failed to give any adduct, the coupling of 4- (trifluoromethyl)benzyl diazoacetate to insulin was quite successful with 75% conversion and a very small amount of bis-insertion (2%) as detected by MALDI (see Figure 4B). A stoichiometric amount of TSPPFeCl was necessary to give 20% conversion with diisopropyl diazomethylphosphonate.
Conclusion
This paper suggests that diazo chemistry is not the perfect conjugation and assembly technology for all applications, but provides a powerful, attractive alternative to conventional chemistry. As a method for applying metal-catalyzed carbenoid transfer reactions for selective modification of proteins, various amino esters were used as models for NH insertion reactions by four different diazo acetates using hydrosoluble porphyrin complex, TSPPFeCl. We conclude the absence of any O-H insertion, neither from methanol nor from water. It also shows a total regioselectivity (tryptophan and tyrosine). As observed with insulin, insertion is regioselective onto the NH2 termini. In contrast, no regioselectivity was noted with cysteine. The introduction of new functional groups on specific sites containing NH groups can modify the stability of the macromolecules, their biological activity and the interactions between them. In the future, the challenge is to optimize all the possible parameters in water and in protic solvents using diazo ester compounds having functions adapted to the desired effects of the bioconjugation, and using catalysts which tolerate the different functions present on the protein.
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Table 1. Ethyl diazoacetate insertion in N-H bonds of amino esters catalyzed by FeTSPPCl and RuTPPSCO in water.a
Catalyst Substrate Time yield Mono/Bis ratio (%)b
1 --- R = Tryptophan 24 0 ---
2 TSPPFeCl Aniline 15 min 43 100:0
3 TSPPFeCl R = Tryptophan 15 min 66 82:18
4c TSPPFeCl R= -CH2PhOH 15 min 71 86:14
5 TSPPFeCl R= -CH2Ph 15 min 62 83 :17
6 TSPPFeCl R= H 15 min 43 68:32
7 TPPFeCl R = Tryptophan 2 min 82 90 :10
8 TPPFeCl R= -CH2Ph 2 min 80 89:11
9 TSPPFeCl Mono-inserted tryp. 24h traces
10d TSPPFeCl Mono-inserted tryp. 1h 48
11e TSPPRuCO R = Tryptophan 4h 56 80:20
Reaction conditions : aCatalyst/substrate/diazo mixture (1:100:100) in 1mL of CBS (pH 10) at 25°C.
bdetermined by 1H NMR. CSolvent: EtOH/CBS (50/50) (pH 10). d 20 equiv of sodium dithionite vs catalyst are added.e Amino ester and diazo dissolved in ethanol are progressively added to a buffer solution (CBS pH 10) containing the catalyst.
Table 2. Insertion of 4-(trifluoromethyl)benzyl diazoacetate (F) and 4-(thiomethyl)benzyl diazoacetate (S) in N-H bond of amino esters catalyzed by TPPFeCl in CH2Cl2.
Diazo Catalyst Substrate time Yield (%)a
1 F TPPFeCl Aniline 2 min 97
2 F TPPFeCl Tryptophan 2 min 88
3 F TPPFeCl Phenylalanine 2 min 87
4 F TPPFeCl Tyrosine 2 min 66
5 S TPPFeCl Aniline 2 min 62
6 S TPPFeCl Tyrosine 2 min 50
7 S TPPFeCl Tryptophan 2 min 67
8 S TPPFeCl Phenylalanine 2 min 63
Reaction conditions: a solution of substrate/diazo/TPPFeCl) mixture (1 :1 :0.01) in 1 ml of CH2Cl2 placed under argon at 25°C was stirred for 2 min.
Table 3. Insertion of 4-(trifluoromethyl)benzyl diazoacetate (F) and 4-(thiomethyl)benzyl diazoacetate (S) in N-H bond of aminoesters catalyzed by TPPSFeCl in water/methanol.
Diazo catalyst Amino ester time yield (%)
1 F TPPSFeCl Tryptophan 10 min 79
2 F TPPSFeCl Phenylalanine 10 min 81
3 F TPPSFeCl Tyrosine 10 min 51
4a F TPPSRuCO Tryptophan 4 heures 45
5 S TPPSFeCl Tyrosine 10 min 33
6 S TPPSFeCl Tryptophan 10 min 51
Reaction conditions: a solution of substrate/diazo/catalyst mixture (1 :1 :0.01) in 1 ml of MeOH/CBS pH 10) (1/1) placed under argon was stirred at 25°C. a lDiazo and amino ester dissolved in MeOH were added to the buffer solution ((CBS pH 10) containing the catalyst and stirred for 4 h.
Tabe 4. Insertion of diisopropyl diazomethylphosphonate in N-H bond of amino esters catalyzed by TSPPFeCl in water/methanol.
Substrate Yield (%)
1 Tryptophan 90
2 Tyrosine 92
3 Phenylalanine 81
4 Aniline 87
5a Aniline 96
Reaction conditions: a solution of substrate/diazo/catalyst mixture (1:1:0.01) in 2 ml of (MeOH:CBS pH 10) (50:50) was stirred at 25°C for 1 h. a solvent : CH2Cl2, catalyst : TPPFeCl
Table 5. Alkylation of N-Terminus of Insulin by TSPPFeCl-Catalyzed Carbenoid Transfer Reactions.
Catalyst (conc.) Diazoa Conversion
(%)
Mono/bis
inserted ratio
1 TSPPRuCO (0.17 mM) EDA (1 mM) 65 60 : 5
2 TSPPRuCO (0.034 mM) EDA (0,25 mM) 14 99 : 1
3b TSPPFeCl (0.034 mM) EDA (0,25 mM) 90 90 : 10 4b TSPPFeCl (0.034 mM) S (0,25 mM) traces
5b TSPPFeCl (0.034 mM) F (0,25 mM) 75 98 : 2
6b TSPPFeCl (0.034 mM) DDMP (0,25 mM) traces
7b TSPPFeCl (0.17 mM) DDMP (0,25 mM) 20 100 : 0
Reaction conditions: the solution of insulin, diazo and catalyst was stirred for 1h at 25°C in a PBS (pH 7.4)/MeCN (7/3) mixture. aEDA = ethyl diazoacetate, F = 4-(trifluoromethyl)benzyl diazoacetate and S = 4- (thiomethyl)benzyl diazoacetate, DDMP = diisopropyl diazomethylphosphonate.b one equiv of cobaltocene (vs catalyst) was added.
Fig. 1. Structure of the catalysts.
Fig. 2. 1H-13C HMBC spectrum of ethyl diazoacetate insertion in N-H terminus bond of tryptophan.
Fig. 3. HPLC chromatograms of racemic (A) and optically active (B) phenylalanine N-H insertion with ethyl diazoacetate.
NH-CH
2
*
CH
CH
2-
*
CH
NH
∗ H N
O O O
O
25 50 75 100
Minutes -46
0 100 200 300 400 500 mA U
78.612
II + II - WI:16
25 50 75
Minutes -22
0 50 100 150 200 mA U
70.207 78.727
II + II - WI:16 WI:32
Fig. 4. MALDI-TOF of coupling ethyl diazoacetate (A) and 4-(trifluoromethyl)benzyl diazoacetate (B) to bovine insulin
Fig. 5. Mass spectrum of insulin after ethyl diazoacetate coupling.
L H
Q N
V F
b5*
b4*
b3*
a2*, b2*
a1*
Fig. 6. 1H NMR of insulin after A diazoacetate insertion and, B unmodified insulin.
Scheme 1. Synthesis of benzyl diazoacetate derivatives.
Scheme 2. Catalytic insertion of ethyl diazoacetate in N-H bond of amino esters.
Scheme 3. Catalytic insertion of benzyl diazoacetate derivatives in N-H bond of amino esters.
Scheme 4. Catalytic insertion of diisopropyl diazomethylphosphonate derivatives in N-H bond of amino esters.
